Add flag to dart2js to configure whether to use the kernel lowering.

pkg/compiler/lib/src/commandline_options.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.cmdline.options;
 
 /// Commandline flags used in `dart2js.dart` and/or `apiimpl.dart`.
 class Flags {
   static const String allowMockCompilation = '--allow-mock-compilation';
   static const String allowNativeExtensions = '--allow-native-extensions';
@@ skipping 26 matching lines @@
   static const String showPackageWarnings = '--show-package-warnings';
   static const String suppressHints = '--suppress-hints';
   static const String suppressWarnings = '--suppress-warnings';
   static const String terse = '--terse';
   static const String testMode = '--test-mode';
   static const String trustPrimitives = '--trust-primitives';
   static const String trustTypeAnnotations = '--trust-type-annotations';
   static const String trustJSInteropTypeAnnotations =
       '--experimental-trust-js-interop-type-annotations';
   static const String useContentSecurityPolicy = '--csp';
+  static const String useKernel = '--use-kernel';
   static const String useNewSourceInfo = '--use-new-source-info';
   static const String verbose = '--verbose';
   static const String version = '--version';
 
   static const String conditionalDirectives = '--conditional-directives';
 
   // Experimental flags.
 
   // Considerations about this feature (esp. locations where generalizations
   // or changes are required for full support of generic methods) are marked
@@ skipping 11 matching lines @@
 }
 
 class Option {
   static const String showPackageWarnings =
       '${Flags.showPackageWarnings}|${Flags.showPackageWarnings}=.*';
 
   // Experimental options.
   static const String resolutionInput = '--resolution-input=.+';
   static const String bazelPaths = '--bazel-paths=.+';
 }